1. Technical Field of the Invention
The invention relates generally to the field of integrated circuits and, more particularly, to fusible link programming in SOI technology.
2. Description of Related Art
In integrated circuits including CMOS integrated circuits, it is often desirable to be able to permanently store information, or to form permanent connections of the integrated circuit after it is manufactured. Fuses or devices forming fusible links are frequently used for this purpose. Fuses can also be used to program redundant elements to replace identical defective elements, for example. Further, fuses can be used to store die identification or other such information, or to adjust the speed of a circuit by adjusting the resistance of the current path.
One type of fuse device is “programmed” or “blown” using a laser to open a link after a semiconductor device is processed and passivated. This type of fuse device requires precise alignment of the laser on the fuse device to avoid destroying neighboring devices. This and other similar approaches can result in damage to the device passivation layer, and thus, lead to reliability concerns. For example, the process of blowing the fuse can cause a hole in the passivation layer when the fuse material is displaced.
Another type of fuse device 30, illustrated in plan view in FIG. 1A and cross-section view in FIG. 1B through line A-A′, and cross-section view in FIG. 1C through line B-B′, is based on rupture or agglomeration or electromigration of silicided polysilicon. These type of fuses include a silicide layer 20 disposed on a polysilicon layer 18, overlain by a layer of silicon nitride 24. Contacts 25 are coupled to the silicide layer 20 in a pair of contact regions 22 on either side of a fuse element 27 to provide an electrical connection between the fuse and external components for programming and sensing. FIG. 1A illustrates a top view of the typical shape and includes the fuse element 27 and contact regions 22. FIG. 1B shows a side view of a typical fuse construction in which the polysilicon layer 18 and the silicide layer 20 are provided at a uniform thickness disposed on an oxide layer 10 also of a uniform thickness, and FIG. 1C illustrates a cross-section through the fuse link region 27. Generally, a blanket nitride capping layer 24 is also provided over layers 20 and 22.
The silicide layer 20 has a first resistance and the polysilicon layer 18 has a second resistance which is greater than the first resistance. In an intact condition, the fuse link has a resistance determined by the resistance of the silicide layer 20. In common applications, when a programming potential is applied, providing a requisite current and voltage over time, across the fuse element 27 via the contact regions 22, the silicide layer 20 begins to randomly “ball-up” eventually causing an electrical discontinuity or rupture in some part of the silicide layer 20. Thus, the fuse link 27 has a resultant resistance determined by that of the polysilicon layer 18 (i.e. the programmed fuse resistance is increased to that of the second resistance). However, this type of fuse device can result in damage to surrounding structure and/or suffers from unreliable sensing because of the inconsistent nature of the rupture process and the relatively small change typically offered in the programmed resistance. Further, these types of devices may not be viable for use with many of the latest process technologies because of the required programming potentials, i.e. current flow and voltage levels over a requisite amount of time.
In another type of fuse, the electromigration fuse, a potential is applied across the conductive fuse link via the cathode and anode in which the potential is of a magnitude and direction to initiate electromigration of silicide from a region of the semiconductor fuse reducing the conductivity of the fuse link. The electromigration is enhanced by commencing a temperature gradient between the fuse link and the cathode responsive to the applied potential. Even with this type of fuse, the programming of the fuse is still dominated by the polysilicon material. Since the polysilicon film contains a significant number of imperfections the final resistance has a wide distribution. This sometime results in a programmed fuse from being sensed incorrectly leading to the failure of the chip.
Therefore, a need exists for a programming method and apparatus which reduces the variability of programming inherent in fuses formed on polysilicon. Furthermore it is desirable to reduce the energy required to program the fuse. It is also preferable to have a significant difference in resistance between the programmed and the un-programmed state. Also, there is a need to shrink the area occupied by the support circuitry.